An AC/DC converter converts alternating current (AC), which periodically reverses direction, to direct current (DC) which flows in only one direction. It is typically used in DC power supplies and high-voltage direct current power transmission systems. For example, in a power generator the voltage on the stator is built up on the terminal side by injecting a DC excitation current into the rotor windings while the rotor is rotating. The rotor winding is provided with DC excitation current through an AC/DC converter including a rectifier circuit and a power transformer. The rectifier circuit may comprise three legs, each of the legs connected to a corresponding AC phase and including at least one rectifier which is an essential component in the rectifier circuit for converting an AC voltage to a DC voltage. A rectifier may be either a diode or a thyristor.
In case of a rectifier fails in the rectifier circuit, e.g. a short-circuited or non-conductive diode/thyristor, the rotor system is extremely endangered. Therefore, there is a need to detect a malfunctioned rectifier when the AC/DC converter is operational.
US 2010/0066551 A1 describes a controller for diagnosing a power generation failure for a power circuit in an electric drive. The power circuit includes a rectifier circuit. To determine a failure of one of the diodes in the rectifier circuit that converts a three-phase AC power to DC power, it calculates the RMS phase current values of each of the 3-phases and then compares the ratio of the RMS phase current values of the phases. When the ratio exceeds a threshold, the controller determines that one of the current phases is unbalanced. This determination may be indicative that one of the diode legs of the rectifier circuit has malfunctioned. Additionally, the controller may attempt to isolate the particular diode that has failed. To perform such diagnosis, the controller determines in which of the legs of the rectifier current flow is in an opposite direction as compared to the remaining legs. The controller may perform other permutations of the three phase currents to make this determination. Using this approach, a short-circuited diode can be detected, whilst another type of fault, i.e. a non-conductive diode cannot be detected.
U.S. Pat. No. 5,508,601 discloses a shorted rectifying diode protection system which monitors the exciter field peak current protection circuit and the output generator voltage to sense the presence of the shorted rectifying diode. Using this solution, a non-conductive diode cannot be detected either.